Currently, transmission systems employed in the cable television industry provide two-way transmission of information ( e.g., video, multimedia and/or data) between the headend and a plurality of subscribers. Typically, the headend transmits the information destined for individual subscribers (“downstream information”) in an optical format, via one or more fiber optic links to one or more optical nodes. Each node converts the optically formatted downstream information into electrical signals for distribution, typically via a cable plant having a hybrid fiber/coax (HFC) architecture, to individual subscribers.
In addition to receiving the downstream information, each individual subscriber may generate information in the form of voice, data, or a combination thereof, destined for the headend. En route to other subscribers or service providers, the subscriber-generated information (“upstream information”) is segmented by the coaxial cable plant and passes it to the node for conversion into an optical format for transmission to the headend. The return path frequency band (e.g., 5-40 MHz) associated with the upstream information is often shared by all subscribers served by the same optical node.
Cable service providers are accustomed to low cost hardware and software components. This has typically constrained the technical capability of the equipment employed in the upstream or return path. Traditionally, this has caused the return path to play a limited role in furnishing, for example, such services as pay-per-view (PPV) or video-on-demand (VOD). In particular, the laser transmitters employed in the optical nodes that transmit information to the headend have been relatively simple, low cost designs. For example, it is well known that the gain of a return laser transmitter fluctuates as a result of environmental changes such as temperature changes and may exhibit a loss of performance due to aging. In conventional systems, the gain or optical power of the return path laser was only stabilized from typical changes arising from temperature fluctuations. Even in this case, the stabilization techniques that were employed were limited to techniques that did not take into account the particular characteristics of the individual laser. They also did not monitor the actual signal drive levels and laser output power to make real-time adjustments for other environmental changes. These laser stabilization circuits were often based on the typical performance of a large population of measured lasers.
The demand from consumers to support interactive applications through cable television services has greatly increased in recent years and this increase is expected to continue. This increased level of services demands a commensurate increase in cable television network speed and performance along the return path, which places more stringent requirements on the return path laser transmitter in an HFC transmission system such that they have a better parametric stability with respect to environmental changes.